cnp3-ebook/principles/transport

Exit Zen

9 ../../principles/transport.rst:45

	English Bob : `Hello` 13/10000 · 13
10 ../../principles/transport.rst:46

	English Alice : `What time is it ?` 27/10000 · 27
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	English Bob : `11:55` 13/10000 · 13
12 ../../principles/transport.rst:48

	English Alice : `Thank you` 19/10000 · 19
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	English Bob : `You're welcome` 22/10000 · 22
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	English Such a conversation succeeds if both Alice and Bob speak the same language. If Alice meets Tchang who only speaks Chinese, she won't be able to ask him the current time. A conversation between humans can be more complex. For example, assume that Bob is a security guard whose duty is to only allow trusted secret agents to enter a meeting room. If all agents know a secret password, the conversation between Bob and Trudy could be as follows : 443/10000 · 443
15 ../../principles/transport.rst:58 • ../../principles/transport.rst:64

	English Bob : `What is the secret password ?` 37/10000 · 37
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	English Trudy : `1234` 14/10000 · 14
17 ../../principles/transport.rst:60

	English Bob : `This is the correct password, you're welcome` 52/10000 · 52
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	English If Alice wants to enter the meeting room but does not know the password, her conversation could be as follows : 111/10000 · 111
19 ../../principles/transport.rst:65

	English Alice : `3.1415` 16/10000 · 16
20 ../../principles/transport.rst:66

	English Bob : `This is not the correct password.` 41/10000 · 41
21 ../../principles/transport.rst:68

	English Human conversations can be very formal, e.g. when soldiers communicate with their hierarchy, or informal such as when friends discuss. Computers that communicate are more akin to soldiers and require well-defined rules to ensure a successful exchange of information. There are two types of rules that define how information can be exchanged between computers : 361/10000 · 361
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	English Syntactical rules that precisely define the format of the messages that are exchanged. As computers only process bits, the syntactical rules specify how information is encoded as bit strings. 191/10000 · 191
23 ../../principles/transport.rst:71

	English Organization of the information flow. For many applications, the flow of information must be structured and there are precedence relationships between the different types of information. In the time example above, Alice must greet Bob before asking for the current time. Alice would not ask for the current time first and greet Bob afterwards. Such precedence relationships exist in networked applications as well. For example, a server must receive a username and a valid password before accepting more complex commands from its clients. 538/10000 · 538
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	English Let us first discuss the syntactical rules. We will later explain how the information flow can be organized by analyzing real networked applications. 149/10000 · 149
25 ../../principles/transport.rst:75

	English Application-layer protocols exchange two types of messages. Some protocols such as those used to support electronic mail exchange messages expressed as strings or lines of characters. As the transport layer allows hosts to exchange bytes, they need to agree on a common representation of the characters. The first and simplest method to encode characters is to use the :term:`ASCII` table. :rfc:`20` provides the ASCII table that is used by many protocols on the Internet. For example, the table defines the following binary representations : 542/10000 · 542
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	English `A` : `1000011b` 16/10000 · 16
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	English `0` : `0110000b` 16/10000 · 16
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	English `z` : `1111010b` 16/10000 · 16

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